FIG. 4 shows a conventional micromechanical comb structure as one part of a conventional capacitive, laterally measuring acceleration sensor. This conventional acceleration sensor has a seismic mass 3 which is movably connected to an anchoring arrangement 2 by spiral springs 1.
A first stationary comb structure 11a and a second stationary comb structure 11b are arranged in relation to comb structures 8a and 8b provided on movable seismic mass 3 so that prongs 111 of first stationary comb device 11a and prongs 112 of second stationary comb device 11b are arranged in pairs next to prongs 80 and 81 of comb devices 8a and 8b in order to form measuring capacitors.
Capacitively measuring acceleration sensors of this type are usually made up entirely of identical comb structures 8a, 8b, 11a, 11b with prongs of the same length and the same shape.
In order to evaluate the small changes in useful capacitance of the measuring capacitors, a high frequency electrical signal of 20 kHz to 2 MHZ, for example, is applied to the measuring capacitors. The spectra of rectangular-pulse, triangular-pulse and similar evaluation signals have many even higher frequencies.
The fundamental mode, i.e., the mechanical resonant frequency of the element (intentional excursion for measuring an acceleration) of sensitive direction 6, ranges between a few 100 Hz and a few kHz to around 20 MHZ, depending on the acceleration range.
Because comb structures 8a, 8b, 11a, 11b are free-standing, they can also move independently, thus producing a further, higher-frequency mechanical mode 9, known as the "prong mode". The natural resonance of a prong ranges from 20 kHz to 1 MHZ, depending on the geometric shape, so that it lies precisely within the range of the electrical evaluation frequency.
The electrical excitation voltage is known to produce electrostatic forces between the capacitor plates. When the natural frequencies of the prongs coincide with the electrical evaluation frequency or a spectral component thereof, resonant vibrations are induced in the prongs, which can produce considerable evaluation errors because the electronics cannot distinguish between a natural vibration and a vibration due to the effect of an external acceleration. These types of effects can also greatly influence the temperature responses of the sensors, since the oscillator frequencies are not thermally stable.
FIG. 5 shows a schematic representation of a second design of a conventional micromechanical comb structure of an acceleration sensor.
FIG. 5 shows the reference numbers which designate the same components as shown in FIG. 4. The second conventional design shown in FIG. 5 additionally provides third and fourth stationary comb structures 12a, 12b so that, as in the design shown in FIG. 4, first and second measuring capacitors 4, 5 are provided for differential measurements between movable comb structures 8a, 8b of seismic mass 3 and the stationary comb structures.
FIGS. 3a and 3b show a Bode diagram for the excitation of an acceleration sensor having a conventional comb structure illustrated in FIG. 4 or 5. In particular, FIG. 3a shows the amplitude plotted against the excitation frequency, and FIG. 3b shows the phase plotted against the excitation frequency.
In FIGS. 3a and 3b, numeral 16 represents the resonant frequency of the working mode, numeral 17 represents the resonant frequency of the prongs or fingers, and numeral 18 represents the open-loop gain.
Especially in the case of closed-loop measurement methods, also known as closed-loop position control or force compensation, a closed loop can become unstable in the typical comb structures, as illustrated in FIGS. 3a and 3b. Because the resonance sharpness of the prongs extends above the 0 dB line, the loop exhibits unstable behavior at points 17 and 17' while closing, i.e., it no longer has any amplitude reserves. The loop could be closed with less loop gain, but this would no longer have any advantage over the open-loop measurement method.
An attempt to solve this problem was made with the ADLX50 acceleration sensor developed by Analog Devices, in which a low pass with a cut-off frequency of around 100 Hz was connected on the line side. Although this closes the loop, it does so only at a sufficiently low frequency.
FIG. 6 shows a schematic representation of a conventional micromechanical comb structure of a micromechanical drive. In FIG. 6, the same reference numbers designate the same components, or components with the same functions, as shown in FIGS. 4 and 5. In addition, numeral 13 designates a movable mass, numeral 14 designates a desired direction of movement, and numeral 15 designates a parasitic prong mode. As shown in FIG. 6, prongs 80 are drawn between prongs 111 against the resistance of spiral spring 1 in driving direction 14.
The disadvantages regarding the excitation frequency response described above apply in the same or similar manner to capacitive comb drives of this type, as illustrated in FIG. 6.